Junction ionic

Thus pectins in muro contain most elements of the cable model but have additional features due to esterification (acetyl- as well as methyl-) and branching. The ionic junction zones are similar to those of calcium pectate gels in vitro but also contam methyl-esterified junctions, and most of the single chains probably have a relatively high degree of methyl-esterification. 

The internal solution Is in contact with the electrolyte of the system studied via a porous material. The latter Is essential because It ensures contact is made with a solution having a fixed, well known concentration during the experiment. Nevertheless, it introduces an ionic junction between the Internal solution and the electrolyte of the system being studied. This can be the cause of errors on the potentials measured, which are generally difficult to estimate. However, when certain experimental precautions are taken, these errors are considered insignificant most of the time (see section 3.4.2.2 and appendix A.hl). 

Each of the five conducting volumes inside the electrochemical chain is equipotential. However, there is a potential difference at each interface (in this simplified representation the thickness is assumed to be insignificant), including for ionic junctions. 

For instance, the ionic junction between solid silver chloride, which is an ionic solid where conduction is only due to Ag" ions, and an aqueous solution containing Ag" ions, is permeable to these ions. This is the case despite the fact that there is a difference between the interactions of these Ag ions with the surrounding medium in the aqueous solutions compared to in the AgCI crystalline network. 

Fora system which starts in equilibrium at open circuit and with negligible ionic junction voltages, the word overpotential usually refers to the difference between the interfacial voltages observed with and without current flow. In connection with the general definition , the ohmic drop terms are therefore excluded from overpotential terms. It all comes down to being able to imagine that one can place two references infinitely close to each of the interfaces. The following equation is therefore commonly used ... 

Subsequently, this chapter will only describe situations of this type. Notably, what will not be touched upon are electrochemical systems in which the ionic junction voltages undergo large changes when there is a current flow. 

F A lithium battery is an example of an electrochemical chain with no ionic junction which can be considered to be in equilibrium at open circuit ... 

Equilibrium is rapidly reached and the ionic junction voltage is given by ... 

As for a junction where several species can be exchanged, the ionic junction voltage in equilibrium relates to the differences between the chemical potentials of each charged species, based on the following equations ... 

Numerous applications have been developed in the field of chemical analysis using potentiometric measurements as indicators, including the production of potentiometric sensors and titration devices. In this chapter, we will focus on the defining principles of these potentiometric methods at zero current when these systems are in thermodynamic equilibrium, which is not necessarily true for all potentiometric measurements. In particular, the following description is confined to electrochemical cells with no ionic junction. In practice, these results will also be applied to many experimental cases in which ionic junction voltages can be neglected . 

This section studies systems without any ionic junction. Depending on the particular redox couples in question, this category may correspond to an experimental system or to a system which cannot be built, yet whose thermodynamic characteristics can still be defined nonetheless. 

Appendix A.3.2 addresses these questions in more thorough detail even if the ionic junction voltage is negligible in numerical terms, it may nonetheless be important to consider it from a fundamen tal point of view. 

To take a more general case, let us imagine an electrochemical cell, which may possibly be fictive, where the sum of the ionic junction voltages is zero. This would give the following equations ... 

V For example, the standard emf of the following cell, at pH= 0, can be given using thermodynamic data (assuming the ionic junction voltage is zero ) ... 

By definition, the potential of a redox couple vs SHE is the emf of the Active electrochemical cell, whereby the working electrode is in the half-cell involving the redox couple in question. The counter-electrode is the standard hydrogen electrode at the same temperature. The terminals are made of the same metals and the sum of the possible ionic junction voltages are considered to be equal to zero. In this case we therefore have E,she =... 

In the case of solutions separated by a membrane or a porous material, the equilibrium state corresponds to a perfect mbtture of the two solutions. However, it takes a very long time to reach this state, if the porosity and/or the geometric configuration of the interfacial zone are well chosen (see section 4.4.2). In such a case, the key phenomenon to consider therefore is the fact that the quasi-steady state is rapidly reached. This latter state has different solutions compositions and non-zero junction voltages. From an experimental point of view, the aim is to minimize this quasisteady-state ionic junction voltages. Different examples are outlined in appendix A. 1.1. 

Appendix A. 1.1 highlights the points addressed in this paragraph by giving a selection of different situations where the ionic junction voltages are numerically estimated. 

For certain electrochemical systems it is possible to find experimental conditions which minimise the interactions between the anode and the cathode. Both electrodes remain related to each other since they are crossed by the same current, yet the difference is that the mass transport phenomena occurring at both interfaces do not interact with each other. This type of scenario, which is typically sought after in analytical experiments, is explored in this paragraph, focused exclusively on describing one single electrochemical interface . Moreover, it is worth noting that the same approach can be applied to any interface, such as for instance an ionic junction. 

On the other hand, the current flow disrupts the interface in the case of electrochemical interfaces and ionic junctions. The voltage across the interface (or the junction voltage for an ionic junction) is generally different from that observed at open circuit. It depends a priori on the parameters of the system and deviates from a linear law, such as Ohm s law. Most often it can be assumed that the double layer thickness is much lower than that of the diffusion layers . Typically, one ends up with the following ... 

The simplest electrochemical cell is a cell where both electrodes are in contat with the same electrolyte. This is called a one-compartment cell, with no ionic junction, in industrial-size cells, one cannot generally overlook the presence of convection phenomena, either natural or forced. In this section we will focus on the case of a closed cell, where convection can be disregarded. This for example is the case with either a solid-type electrolyte, a gel or polymer electrolyte, or a small volume of liquid electrolyte, typically with less than 1 mm between both interfaces. Then we will describe a one-compartment cell with forced convection, which relates to Industrial cells with electrolyte circulation (open systems). 

A. 1.1- Liquid ionic junction voltage without current... 

To fix the potential reference, one needs to fix the chloride ion concentration in the reference s inner soiution (see section 1.5.1.2). However, this concentration is not the only ruling factor to be taken into account designing a high-quality reference electrode, as shown when comparing the four examples outlined below (the ideal reference electrode should have a zero ionic junction voltage). The nature of the solution P being studied (here it is HCI at pll= 2) also has an impact on the junction voltage. However, in an experimental situation it is much easier to adapt the composition of solution a... 

The second example deals with a Daniell cell, and shows to what extent the solution that is contained within the salt bridge has an impact on the overall ionic junction voltage. Here the voltage is the algebraic sum of two liquid Junction voltages, illustrated by the following electrochemical chain ... 

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